Large field of view protection optical system with aberration correctability for flat panel displays

ABSTRACT

An exposure system for manufacturing flat panel displays (FPDs) includes a reticle stage adapted to support a reticle. A substrate stage is adapted to support a substrate. A reflective optical system is adapted adapted to image the reticle onto the substrate. The reflective optical system includes a primary mirror including a first mirror and a second mirror, and a secondary mirror. The reflective optical system has sufficient degrees of freedom for both alignment and correction of third order aberrations when projecting an image of the reticle onto the substrate by reflections off the first mirror, the secondary mirror, and the second mirror.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to projection optical systems usedin printing circuit patterns during the manufacture of large flat paneldisplays (FPD), and more particularly, to an optical design form that isrelatively compact, provides aberration and magnification correction andfacilitates a high FPD production rate.

[0003] 2. Related Art

[0004] The manufacture of a liquid crystal display, or a flat paneldisplay (FPD) involves a manufacturing process that is similar to thatused in the integrated circuit (IC) industry where computer chips areproduced. An exposure system is used to project an image of a reticlecontaining a circuit pattern so as to expose a photo resist coatedsubstrate. The actual circuit is created after the exposed substrate isprocessed using standard microlithographic processes. Depending on theparticular FPD design this exposure process may be repeated many timeson one substrate using reticles with different circuit designs. When allthe exposures and microlithographic processing steps have been completedso the desired circuit pattern has been created, the substrate isintegrated with other components to create a flat panel display screen.

[0005] Although FPDs have been in production since the late 1980s, thecurrent size requirement are for FPDs of up to 42 inches diagonal, with54 and 60 inches diagonal under development. This places severerequirements on the optics used in the projection optical system.Specifically, many existing optical design forms, if scaled up to 42inch (and larger) FPD manufacturing size, become unreasonably large,especially from an optical manufacturing and packaging perspective.

[0006] Two different imaging processes are conventionally used tolithographically print circuits on flat panel display screens.Lithography tools described in U.S. Pat. No. 5,625,436, U.S. Pat. No.5,530,516, U.S. Pat. No. 4,769,680 and U.S. Pat. No. 5,710,619 createthe full circuit by stitching together images of small areas of the FPDcircuit design. While acceptable quality and cost FPD screens up to 18inches have been produced using the stitching imaging technique, theerrors inherent in stitching have resulted in a very low product yieldduring manufacturing of larger displays and marginally acceptablequality. Because of the low yield, the production cost for large FPDtelevisions has been at an unacceptably high level from the FPDmanufacturers' marketing perspective. As a result of the residualstitching errors the quality level of FPD televisions has not been thatmuch better than conventional televisions for consumers to justify thehigh cost of an FPD television.

[0007] The major problem encountered in stitching is that the adjacentsmall images that create the full FPD circuit pattern are not aligned toeach other. There are many sources of pattern alignment errors inoptical imaging configurations. However, most of the errors are relatedto the imaging process requiring the use of multiple lens systems andmasks. Misalignment errors result in electrical connection problems inthe FPD circuit and/or an image on the screen that has visuallydispleasing discontinuities. No good solutions have been found tocompletely eliminate the problems associated with stitching. As aresult, FPD manufactures prefer full field imaging or scanning systemsto any optical design configuration that employs stitching.

[0008] Optical designs, operating at 1× or at some other magnification,compatible with producing 42-inch full field scanners, require verylarge lenses and/or mirrors. To print 42-inch FPDs, the lithography toolmust have a minimum slit height of about 525 mm. (Note that while in theUnited States, screen dimensions are usually specified using the Englishsystem, while optical design and tool dimensioning is usually done inmetric.) The requirement that the optical design form be telecentricresults in at least one element in the optical design being 525 mm indiameter and more, typically at least 1,200 mm for the FPD manufacturerspreferred optical design form.

[0009] For 1× refractive and catadioptric design forms, a minimum of adozen and as many as several dozen lenses and mirrors are required. Itis extremely difficult and very costly to achieve the opticalperformance required for printing 42 inch and larger FPD circuits usingoptical designs with refractive elements due to chromatic aberration inthe refractive elements and problems with index of refractionhomogeneity in the lens material. Even if an optical design is optimizedto minimize chromatic aberration, there is a practical limit on theusable spectral bandwidth of the source. The reduced usable spectralbandwidth results in less available light to expose the photoresist. Asa result, refractive and catadioptric manufacturing tools produce fewerFPDs per hour then optical design forms that do not suffer fromchromatic aberration. The lower production rate results in higher costsfor FPD televisions.

[0010] In addition to cost disadvantages, the image quality anddistortion of refractive and most catadioptric design forms arecompromised by the lens material's inhomogeneity, which degrades imagequality and introduces distortion. While lens material inhomogeneityerrors can be partially compensated for during glass production, or inthe lens optical fabrication process, both methods add significantcosts. In optical design, a system optimized to minimize the physicalsize of the lenses has many elements, while a design optimized-to havejust a few elements will have very large elements. However, in either ofthose scenarios, the total thickness of glass required in both designswill result in unacceptable inhomogeneity-related problems. As a result,optical design forms for tools to produce FPDs of approximately 24inches and larger, which make extensive use of refractive materials, areextremely expensive to manufacture or cannot be built because thequality level glasses are not readily available.

[0011] Reflective optical design forms operating at 1× magnification aresuccessfully being used in the microlithography industry, including theproduction of FPDs. An ASML Micralign design form first described inU.S. Pat. No. 3,748,015, which is shown in FIG. 1A, has been used inmanufacturing of 32 inch FPD. The design in FIG. 1A has two sphericalmirrors, a primary concave mirror 101, and a secondary convex mirror102. Note that, as shown in FIG. 1A, the primary mirror is used as areflector twice. A reticle 103, that has the desired FPD circuit patterndrawn on it, is positioned off axis with respect to the optical system'soptical axis. The image of the reticle 103 is projected onto a substrate104 located symmetrically on the opposite side of the optical axis asthe reticle. In this design form, the two spherical mirrors areconfigured to have good image quality and low distortion over an annularfield (an annular field optical design concept is described in U.S. Pat.No. 3,821,763). Aberrations in this design are corrected by usingconcentric optical surfaces, selecting the surface radii of curvatureswith specific relationships and having the reticle and the FPD substratesymmetrically arranged relative to the optical system. Specifically, theradius of curvature of the convex mirror is one-half of the radius ofcurvature of the concave mirror. (Note that it is also possible to usetwo convex and one concave mirrors instead of two concave and oneconvex, but such a design is considerably more difficult.)

[0012] Using these optical design principles results in the opticalsystem being naturally corrected for the aberrations spherical, coma anddistortion. There can be a substantial amount of astigmatism with thisdesign form, with the amount dependent on the reticle size and thenumerical aperture at which the system is operating. The ability tocorrect the astigmatism is extremely important because for this opticaldesign form the astigmatism is what limits the usable slit width, whichin turn determines the production rate for the FPD substrates.Astigmatism can be corrected to a limited extent by small deviations ofthe surfaces from concentricity, or by a small change in the convexmirror radius of curvature.

[0013] While this design form is capable of meeting the opticalperformance requirements for printing 32 inch FPDs, the image qualityand distortion is marginal at best when compared to the particularresolution and overlay requirements typically needed for a 42 inchdisplay. Because of the large increase in astigmatism when imaging a 42inch display, the usable slit width is unacceptably small from theproduct production rate perspective, which will be discussed in thefollowing sections. Also, scaling the 32 inch design to print 42 inchFPD results in one of the mirrors in the optical system being a minimumof 1.2 meters in diameter. This size mirror presents opticalmanufacturing challenges and very difficult packaging problems both fromit's physical size perspective and its approximately 700 kg weight.Also, with this size and weight optic, problems with mounting,alignment, and gravity-induced distortions are generally encountered. Incomparison, the largest mirror in the tool used to manufacture 32-inchFPD is about 800 mm in diameter and weighs on the order of 200 kg.

[0014] In the two-mirror approach of FIG. 1A, an arcuate-shaped regionon the reticle is formed on the FPD substrate, as illustrated in FIG.1B. In FIG. 1B, A is sable slit width. Optical performance is acceptableat any point within the slit width A. B is the center of slit. Opticalperformance gets worse on either side of the center line B. The rate ofperformance falloff generally increases exponentially with increaseddistance from the slit center line B. C is the slit height. An FPD has adiagonal dimension from 42 to 60 inches. Aspect ratio (length to width)is generally the industry standard of 16:9. A larger FPD diagonaldimension results in an increased slit height. Slit heights range for550 mm for a 42 inch diagonal to 775 mm for a 60 inch diagonal. Inoperation the slit is scanned from left to right (or vice-versa) toprint the FPD circuit pattern. At any point in time only the areadefined by the slit is being exposed on the FPD.

[0015] To image the full circuit pattern of the FPD substrate, thearcuate-shaped field of view is scanned across the full width of thereticle. This creates an image of the circuit pattern on the photoresistcoated substrate. The height of the arcuate shape is designed to be thesame as the vertical axis of the FPD screen, which for a 42-inch screenis about 525 mm. This enables a circuit pattern to be imaged on thesubstrate in a single scan. For a 42 inch FPD the screen width is about930 mm.

[0016] The width of the arcuate surface depends on the residualaberrations of the optimized optical design. To achieve a highproduction rate, a large slit width is desired. Larger slit widthsresult in more photons reaching the photoresist per unit time. A greaternumber of photons per unit time enables a shorter exposure time whenprinting the circuit pattern, thus enabling more FPD substrates to beprinted per hour. Based on the typical power level in a FPD tool sourcesystem, an arcuate width of at least 5 mm is desired.

[0017] It is important to note that as the FPD size increases, theresidual aberrations increase in any 1× optical system. Not only doesthe magnitude of the residual aberrations increase with FPD size, butaberrations which could be previously ignored because of their smallsize now reach a magnitude where they must be considered in the opticaldesign optimization process. The aberration increase is not linear withFPD size. The aberrations of concern increase with square, fourth andsixth power of the FPD size.

[0018] In order to meet the image quality and distortion requirementsnecessary to print FPD circuit patterns, these aberrations must bereduced in magnitude to an acceptable level. Adjusting the variousparameters, such as the optical surface curvatures, surface shape,optical element spacing, aperture stop location, etc. can control themagnitude of the aberrations. For those familiar in the art of opticaldesign, it is a well known principle that the number of differentaberrations that can be corrected is directly related to the number ofparameters, or degrees of freedom, that are available for adjustment.For example, six degrees of freedom enable six aberration aberrations tobe corrected (“corrected” means that the magnitude of an aberration canbe improved). However, these same degrees of freedom are needed tocontrol other aspects of the optical design, such as the first orderdesign characteristics magnification, focal length, back workingdistance, etc. As a result, after accounting for the degrees of freedomneeded to control the first order design characteristics, only one ortwo of the degrees of freedom out of the original six may be availableto correct aberrations. Unfortunately, some of the optical designvariables that are considered “degrees of freedom” have very littleimpact on the relative magnitude of the aberrations. While an opticalsystem may have six degrees of freedom, only four of those variables mayimpact the aberrations' magnitudes in a meaningful way. Because of thisthe number of degrees of freedom is therefore an important factor in anoptical design form. From the aberration correction perspective, anoptical design form that has many degrees of freedom will enablesuperior optical performance to be achieved as compared to a design formthat only has a few degrees of freedom.

[0019] In designing the conventional two-mirror system described in U.S.Pat. No. 3,748,015, there are only 9 degrees of freedom: spacing betweenobject plane and concave mirror 101; radii of curvatures of concavemirror 101 and convex mirror 102; x and y tilt of concave mirror 101;concave mirror 101 to convex mirror 102 distance; x and y tilt of convexmirror 102; and concave mirror 101 to image plane distance. (Forspherical mirrors, a lateral displacement, or decenter, is theequivalent to tilting the surface.) During the optical design process,six of these degrees of freedom are needed to control numericalaperture, magnification, focal length and alignment related errors. Thisleaves three variables, which is insufficient to control all theaberrations plus any other optical performance considerations, such astelecentricity.

[0020] Accordingly, for the manufacture of large-scale FPD's, it isdesirable to have an optical design form that enables relatively smalloptics to be used in the design and to have sufficient degrees offreedom available to correct all the critical aberrations and relatedoptical performance considerations.

SUMMARY OF THE INVENTION

[0021] The present invention is directed to a large field of viewprojection optical system with magnification and aberrationcorrectability for large flat panel display manufacturing thatsubstantially obviates one or more of the problems and disadvantages ofthe related art.

[0022] Accordingly, there is provided an exposure system formanufacturing flat panel displays (FPDs) including a reticle stageadapted to support a reticle. A substrate stage is adapted to support asubstrate. A reflective optical system is adapted to image the reticleonto the substrate. The reflective optical system includes a primarymirror divided into a first mirror and a second mirror, and a secondarymirror. The reflective optical system has sufficient degrees of freedomfor both alignment and correction of third order aberrations whenprojecting an image of the reticle onto the substrate by reflections offthe first mirror, the secondary mirror, and the second mirror.

[0023] In another aspect there is provided an exposure system formanufacturing FPDs including a substrate stage adapted to support an FPDsubstrate. A reflective optical system adapted to image a reticle ontothe FPD substrate includes only a first mirror, a second mirror, and asecondary mirror as its curved reflective elements. The reflectiveoptical system has at least 14 degrees of freedom.

[0024] In another aspect there is provided a unit magnificationringfield optical system for FPD manufacturing including a mask havingthe FPD circuit pattern. A projection optical system for projection ofan image of the mask onto a substrate includes a first concave mirrorand a second concave mirror, and a convex mirror. The concave mirrorshave their centers of curvature substantially coincident with theconcave mirror, and are substantially symmetrically positioned withrespect to a center of the convex mirror. A radius of curvature of theconvex mirror is one-half of radii of curvature of the concave mirror.In these design forms, the mirrors are aspherical with centers ofcurvatures of the mirrors being substantially coincident and the convexmirror radius of curvature is nominally one half that of the concavemirror. This reflective optical system form has 10 more degrees offreedom then the previously described two aspheric mirror design form.These extra degrees of freedom are used to improve the projection opticsperformance as compared to a two spherical mirror design in U.S. Pat.No. 3,748,015. The extra degrees of freedom available due to splittingthe single concave mirror in to two mirrors is very important whenadjustments to object and/or image plane location and magnification arerequired when using the optical system. Any adjustment to the objectand/or image and magnification will also result in unwanted aberrationsbeing introduced. These aberrations can be corrected for by using one ormore of the extra degrees of freedom available by splitting the concavemirror.

[0025] In another aspect there is provided unit magnification ringfieldoptical system for FPD manufacturing including a mask having a circuitpattern for the FPD. A projection optical system for projection of animage of the mask onto a substrate includes a first convex mirror, asecond convex mirror and a concave mirror. The first and second convexmirrors have their centers of curvature substantially coincident with acenter of curvature of the concave mirror. The first and second convexmirrors are substantially symmetrically positioned with respect to acenter axis of the concave mirror. A radius of curvature of the concavemirror is one-half of radii of curvature of the convex mirrors.

[0026] In another aspect there is provided an exposure system formanufacturing FPDs including a source of electromagnetic radiation, areticle mounted on a reticle stage, and a reflective optical systemadapted to image the reticle onto a substrate that is mounted on asubstrate stage. The reflective optical system includes only threepowered reflective surfaces, including a first mirror and a secondmirror, and a secondary mirror. The reflective optical system hassufficient degrees of freedom for both alignment and correction of thirdorder aberrations.

[0027] Additional features and advantages of the invention will be setforth in the description that follows. The advantages of the inventionwill be realized and attained by the structure particularly pointed outin the written description.

[0028] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory and are intended to provide further explanation of theinvention.

BRIEF DESCRIPTION OF THE FIGURES

[0029]FIG. 1A shows a conventional two-mirror projection optical systemused in the FPD manufacture.

[0030]FIG. 1B shows an arcuate-shaped region on the reticle formed onthe FPD substrate.

[0031]FIG. 2 shows the arcuate shaped slit shape characteristic of theannular field optical design form in FIG. 1.

[0032]FIG. 3 shows a projection optical system of the present invention.

[0033]FIG. 4 shows the actual area used on concave mirror.

[0034]FIG. 5 illustrates the reduced size of the mirrors in the newoptical design form.

[0035]FIG. 6 shows a projection optical system of the present inventionwith corrector lenses.

[0036]FIG. 7 shows a ray tracing diagram of one example of an opticalsystem of the present invention.

[0037]FIG. 8 shows the optical performance of the optical system of FIG.7 in graphical form.

[0038]FIG. 9 shows a ray tracing diagram of one example of an opticalsystem of the present invention.

[0039]FIG. 10 shows the optical performance of the optical system ofFIG. 9 in graphical form.

DETAILED DESCRIPTION OF THE INVENTION

[0040] Reference will now be made in detail to the embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Whereever possible, the same reference numeral is used todesignate the same element in different figures.

[0041]FIG. 2 illustrates how, in the conventional two-mirror projectionsystem of FIG. 1, only a small portion of the primary mirror 101 isactually used. The fact that only a small portion of the mirror 101 isused provides the rationale for splitting the mirror 101 into twoseparate mirrors, which are smaller than the single primary mirror wouldotherwise need to be. In the present invention, with the primary mirrorbeing split in two, it is possible to adjust the properties of the imagewithout affecting the properties of the object.

[0042] A new optical design form for a 1× lithography tool is describedthat has significant benefits over current optical systems, especiallythose used in printing flat panel displays (FPDs) that are 42 inchesdiagonal in size and larger, for example, 54 or 60 inches diagonal. Thenew design form has benefits in size, weight, optical manufacturability,mechanical mounting, optical system alignment, compensating adjustmentoptions, packaging and cost over conventional designs.

[0043] The new optical design form employs aspheric optical surfaces.The aspheric surfaces enable superior resolution, optical distortion andtelecentricity as compared to the prior art spherical surface opticaldesign forms. The new design uses three mirrors—two concave and oneconvex—that are aligned to each other so their centers of curvature arenominally coincident. The reticle and substrate are nominally positionedalong a plane perpendicular to the optical axis at the location of themirrors' centers of curvature. By making the centers of curvatures ofthe mirrors substantially coincident, the three mirror design iscorrected over an annular field of view for all the third orderaberrations with the exception of astigmatism, which is only correctedat one location in the annular field. Third order astigmatism over thedesired annular field, higher order aberrations and telecentricity canbe corrected using the added degrees of freedom that are available whenusing aspheric surfaces. A commercial optical design program such asCode 5™ or Zemax™ can be used to easily calculate by someone familiar inthe art the aspheric terms to correct the aberrations to an acceptablelevel.

[0044] While only one set of aspheric terms for the mirrors will resultin the best optical performance, there are many combinations of asphericterms that will result in acceptable optical performance.

[0045] The individual mirror radii of curvatures, the spacing betweenmirrors and concentric center of curvature condition can be also beadjusted during the optical design process for calculating the asphericterms. Adding these degrees of freedom will reduce the number ofaspheric terms required to achieve the required optical performance.Reducing the number of aspheric terms will result in more opticalmanufacturing companies being able to fabricate the mirrors and lowermirror costs.

[0046] The optical design process is repeated adding the designvariables available with this element. While it is possible to achievesuperior performance by adding correctors to a design optimized usingjust the three mirror, this system will not achieve as good aperformance as a design optimized using all the degrees of freedomavailable simultaneously has important benefits.

[0047] Adjustments of 1-2 mm or so in the relative positions of thecenters of curvature are possible, to correct for aberrations. Theconcave mirrors are symmetrically positioned with respect to the centeraxis of the convex mirror, although adjustments of 1-2 mm or so arepossible here as well, to correct for additional aberrations.

[0048] In addition, by slightly changing the radius of curvature of oneof the concave mirrors, the fifth order astigmatism can be corrected,while still having a system with a level of Petzval curvature thatenables the resolution requirement to be achieved across the desiredfield of view.

[0049] The new optical design forms have at least 10 extra degrees offreedom, as compared to the conventional design form, for full fieldscanning. The extra degrees of freedom can be used to compensate forother errors in the system, such as coating errors, gravity induceddistortion of the optics, optic mount-induced distortion, etc. Ifdesired, the extra degrees of freedom can also be used to relax someoptical specifications for components in the optical system.

[0050] Although an asphere can take the shape of any surface that can bemathematically described, an even or odd high order polynomial istypically used. The following equation is an example of an even highorder polynomial used to describe an asphere:$z = {\frac{{Cy}^{2}}{1 + \left\lbrack {1 - {\left( {1 + K} \right)C^{2}y^{2}}} \right\rbrack} + {Ay}^{4} + {By}^{6} + {Cy}^{8} + {Dy}^{10}}$

[0051] Where z is the departure of the aspheric surface from the basespherical surface sagittal; C is the reciprocal of the spherical surfaceradius of curvature; y is the location on the surface; K is an asphericdeparture term, usually referred to as a conic constant; A, B, C, D aregenerally referred to as the coefficients of each high order term forthe polynomial.

[0052] The equation above is an example of an “even” high orderpolynomial.

[0053] In addition to the mirror surfaces radii of curvature being adesign variable degree of freedom during the optical design process,each surface described by an even order polynomial can have the addeddegrees of freedom K, A, B, C, D, etc. As a result of having these extradegrees of freedom more aberration can be corrected then in the 1×optical design form described in U.S. Pat. No. 3,748,015.

[0054] The number of aspheric terms used in the design is dependent onthe acceptable optical design residual. The conic constant K, and fourhigher order aspheric coefficients, A, B, C and D, result in acceptableperformance for projection optics used to print 42-inch FPD circuitsalong with the desired object and image locations, magnification andarcuate slit width.

[0055] Because the two concave mirrors can be moved independently it ispossible to make system magnification adjustments which are not possiblein the prior art.

[0056] In this design form the centers of curvatures of the concave andconvex mirrors are substantially coincident and the convex mirror radiusof curvature is nominally one half that of the concave mirror. Inaddition to the radii of curvature of each mirror being a variable, asin the two-mirror design of U.S. Pat. No. 3,748,015, in the projectionoptics optical design, an aspheric component is added to the concavemirror. Different forms and types of asphericity can be added to eachsurface. Each aspheric term added to each reflective surface is anotheroptical design variable. As a result, it is possible to reasonably add 5additional variables to the concave surface in the optical designprocess. The number of aspheric terms used in the design is dependent onthe acceptable optical design residual. The conic constant and fourhigher order aspheric coefficients result in acceptable performance fora projection optical system used to print 42 inch FPD circuits. Thecombination of the mirror radii of curvatures, aspheric surface,spacings and tilts enable the desired object and image locations,magnification and correction of aberrations to an acceptable level.

[0057] Conventionally, 32-inch FPDs are printed using lithography toolswith an 800 mm diameter spherical mirror. The new optical design formallows 42-inch FPDs to be printed with an 800 mm diameter mirror.

[0058] The new baseline design has the same optical performance as thetwo-mirror design currently in use for making 32-inch displays, but withsignificantly smaller mirrors. Mirror size is a critical issue inlithography tools for printing 42 inch FPDs. A two-mirror designrequires one of the mirrors in the system to be >1.1 meter, while thethree mirror design of the present invention has a maximum mirror sizeof about 750 mm (and scaling roughly linearly for 54 and 60 inchdiagonal FPDs).

[0059] Accordingly, a nominally 1× lithographic exposure system formanufacturing flat panel displays (FPDs) includes a reticle stageadapted to support a reticle. A substrate stage holds a substrate onwhich an FPD circuit pattern will be printed. A reflective opticalsystem with a large number of degrees of freedom images the reticle ontothe substrate. The reflective optical system includes a concave primarymirror, a convex secondary mirror and concave tertiary mirror with oneor more of these mirrors having aspheric surfaces.

[0060] In the baseline design form, the two concave mirrors have radiiof curvatures and aspheric profiles that are nominally identical, theconvex mirror radius of curvature is nominally one half that of theconcave mirrors and the centers of curvatures of all the mirrors aresubstantially coincident. FIG. 3 illustrates this optical design form.The exact radii of curvature and type of aspheric surface on the mirrorsurfaces depends on the FPD reticle size and optical performancerequired over the operating slit width. The benefit of adding asphericsurfaces to the optical design is the increase in the number of degreesof freedom available during the optical design process.

[0061] The invention works by splitting into two mirrors what would be asingle concave mirror in the conventional optical design. FIG. 3 showshow this can be accomplished and results in smaller mirrors then in theconventional design. By splitting one large concave spherical mirror inthe design into two mirrors 301 a, 301 b, the sizes of the mirrors 301a, 301 b in the optical design are significantly reduced. Splitting oneoptical surface into two and reducing the size of the elements hasoptical manufacturing benefits, lithography tool size benefits, andoffers increased flexibility in correcting optical system alignmenterrors.

[0062] As may be seen in FIG. 3, the projection optical system of thepresent invention includes the primary concave mirror 301 a, a secondary(convex) mirror 302 and the tertiary concave mirror 301 b. A reticle 303is positioned off axis (i.e., off axis 306). An image of the reticle 303is projected onto a substrate 304.

[0063] In the system shown in FIG. 3, the concave mirrors 301 a, 301 bare aspherical, and are symmetrically positioned with respect to thecenter of the secondary mirror 302. The concave mirrors 301 a, 301 btypically have the same radius of curvature (although this need notalways be the case). The radius of curvature of the secondary mirror 302is about one-half of the radius of curvature of the concave mirrors 301a, 301 b.

[0064] The configuration of FIG. 3 allows the use of smaller mirrorsthan in equivalent unit magnification systems, so that the two mirrors301 a, 301 b replace a single large concave mirror. The centers ofcurvatures of the concave mirrors 301 a, 301 b may be displaced, so asto compensate for the aberrations that result from any magnificationadjustments, from residual optical manufacturing errors, from coatingerrors, or from mount-induced errors. In other words, the availabilityof additional degrees of freedom can be taken advantage of to compensatefor additional aberrations.

[0065] In addition to the design configuration based on the asphericequation above having superior optical performance than the design formin FIG. 1A, the design form in FIG. 3 has the added advantage that bothconcave mirrors are smaller in size and weight then the concave mirrorin FIG. 1A. The benefits of the smaller size mirror include reducedoptical fabrication costs, less unwanted gravity induced distortion tothe mirror surface shape and easier mounting in the lithographic tool.

[0066]FIG. 4 shows the actual area used on concave mirror andillustrates why the two concave mirrors 301 a, 301 b, can replace thesingle concave mirror.

[0067] For each degree of freedom, an aberration can be corrected. Notethat increasing the number of mirrors or the number of aspherical termsused to describe an aspheric surface in the design is directly relatedto the number of degrees of freedom available during the optical designand alignment.

[0068] Also, the aspherization of the mirrors 301 a, 301 b, need not beidentical. The convex mirror 302, when it is polished, has a certaindegree of astigmatism. It is possible to shape the concave mirrors 301a, 301 b so as to introduce the opposite amount of astigmatism,canceling the overall astigmatism out. Alternatively, only one of theconcave mirrors 301 a, 301 b may be purposely mis-aligned, so as tocancel out the astigmatism due to the fabrication errors. The use of thethree-mirror design of FIG. 3 also allows correcting for gravity-induceddistortions, which is difficult to do with a two-mirror design.

[0069] In addition to the mirrors 301 a, 301 b, the secondary mirror 302may also be aspheric.

[0070] From an optical manufacturing perspective, the smaller mirrors301 a, 301 b have several important advantages over the single largemirror design of FIG. 1A:

[0071] Substrate price is impacted by both the total volume and by thesize of the substrate (due to handling, equipment and fabrication time);

[0072] Generating, grinding and polishing time is less because there isless total surface area with the smaller substrates; and

[0073] The size and weight of the mirrors is compatible with standardcommercial optical manufacturing equipment.

[0074] In the lithography tool, the reduced mirror size and rectangularshape results in easier mounting, less gravity induced distortion, andless demanding adjustment mechanisms.

[0075] An additional benefit obtained by splitting the concave mirrorinto two mirrors 301 a, 301 b and having aspheric surfaces includesbeing able to adjust spacings between the three mirrors, which canchange the focal length of the system. In a two-mirror design, adjustingthe focal length introduces spherical aberration. However, in thethree-mirror design, by adjusting the two mirror spacings, most of thespherical aberrations can be removed after a focal length change hasbeen made.

[0076] If the optical coating process introduces spherical aberrationand/or astigmatism, re-spacing and tilting the mirror 301 a or 301 bwill correct these errors.

[0077] If the thermal load on any of the mirrors causes a mirror surfaceto distort with a low order aberration, spacing and/or tilt changes cancorrect the error.

[0078] Magnification in a two-mirror system is adjusted by changing thereticle distance. However, this introduces aberrations. Re-spacing themirrors correct most of the wavefront aberrations.

[0079] Mirror mount induced surface figure errors can be mostlycorrected if they are a low order aberration.

[0080] Optical manufacturing errors that result in the wrong radii ofcurvatures for the mirrors can be mostly compensated for by spacingchanges between the three mirrors.

[0081] If it is determined a refractive or reflective corrector isneeded because of a requirement for a large slit width, and then thecommercial optical design program can be used to calculate the opticalprescription, as would be apparent to one of ordinary skill in the art.Again, there are many combinations of radii of curvatures and asphericcoefficients for the correctors that will result in acceptable opticalperformance. Thus, refractive corrector lenses may be added betweeneither or both the mask and primary mirror and/or the tertiary mirrorand photoresist coated substrate. These correctors result in betterimage quality and distortion and wider operating slit as compared to thetwo aspheric mirror design, as would be apparent to one of ordinaryskill in the art.

[0082] The correctors can be designed to be located at any distancebetween the reticle and primary mirror or tertiary mirror and substrateprovided they do not cause vignetting (a gradual fading of the imagetowards the edges of the image). However, the best locations for thecorrectors are as close to the reticle and substrate that is practicalconsidering mechanical packaging of the optical system. In the baselinedesign form, the two concave mirrors have radii of curvatures andaspheric profiles that are nominally identical, the convex mirror radiusof curvature is nominally one half that of the concave mirrors and thecenters of curvatures of all the mirrors are substantially coincident.The refractive corrector lenses can take the form of flat and parallelsurfaces, one surface flat with the other having an aspheric profileadded to the flat surface, both with aspheric surfaces, meniscus lens,meniscus lens with aspheric surfaces on either or both surfaces. Thecorrector elements on the reticle and substrate sides have nominally thesame optical design prescription. The exact optical prescription for thecorrector element depends on the reticle size, operating wavelength andthe optical resolution and distortion requirements.

[0083]FIG. 5 illustrates a modification of the system of FIG. 3. As maybe seen in FIG. 5, relatively thin, flat glass plates 505, 506 may beadded, either near the reticle 303, or near the substrate 304, or both.The thickness of the glass plates 505, 506 is typically on the order of5-10 mm for a 42-inch FPD manufacturing system (roughly scaling linearlywith FPD size). The glass plates 505, 506 may be used to compensate forvarious aberrations in the optical system, as would be apparent to oneskilled in the art. The thickness of the glass plates 505, 506 dependson overall system design, and the magnitude and type of aberrations thatneed to be corrected.

[0084] Additionally, a meniscus lens 508 or 507 may be used to furthercorrect the aberrations, as shown in FIG. 5. One or both meniscus lenses508, 507 may be used (and may also be used together with the glassplates 505, 506). The meniscus lenses 508, 507 are roughly on the orderof 1-2 cm in thickness for a 42-inch FPD manufacturing system (i.e., adiameter of about 525 mm).

[0085] Reflective corrector mirrors also may be added between either orboth the mask and primary mirror and/or the tertiary mirror andphotoresist coated substrate. The correctors can be designed to belocated at any distance between the reticle and primary mirror ortertiary mirror and substrate, provided they do not cause vignetting.However, the best locations for the correctors are as close to thereticle and substrate that is practical considering mechanical packagingof the optical system. These correctors result in better image qualityand distortion and wider operating slit as compared to the threeaspheric mirror design.

[0086]FIG. 6 illustrates this optical design form. In the baselinedesign form the two concave mirrors have radii of curvatures andaspheric profiles that are nominally identical, the convex mirror radiusof curvature is nominally one half that of the concave mirrors and thecenters of curvatures of all the mirrors are substantially coincident.The reflective correctors have flat surface on which the desiredaspheric prescription is added. The corrector elements on the reticleand substrate sides have nominally the same optical design prescription.The exact optical prescription for the corrector element depends on thereticle size, operating wavelength and the optical resolution anddistortion requirements.

[0087] As shown in FIG. 6, nominally flat mirrors 610, 609 may be usedas reflective correctors. The mirrors 609, 610 may have asphericity, andmay be oriented at an angle of 45 degrees, or some other angle,consistent with their asphericity. Either the mirror 609, or the mirror610, or both, may be used. Alternatively, either the mirror 609, or themirror 610, or both, may be powered mirrors.

[0088] Two examples will be used to illustrate the present invention,although it will be appreciated that an infinite number of examples canbe generated, depending on how the merit function of the optical systemis defined. One of ordinary skill will appreciate that the invention isnot limited to the optical prescriptions below, which may be generatedusing a computer and known optical design software.

EXAMPLE 1

[0089] The table below gives exemplary optical prescription for Example1: 1X Optical Design Example #1 Optimized For Reduced Volume RadiusDistance Conic Aspheric Coefficients Surface (mm) (mm) Material ConstantY⁴ Y⁶ Y⁸ Y¹⁰ Object ∞ 228.3 Air 0.0 0.0 0.0 0.0 0.0 L1 −460.7 30.0 Fused0.0   2.1201 × 10⁻⁹    3.8746 × 10⁻¹⁴ −3.5049 × 10⁻¹⁹   1.1333 × 10⁻²⁴Silica L2 −687.3 1583.3 Air 0.0 0.0 0.0 0.0 0.0 M1 −1186.8 −721.8Reflection −0.9452 −1.1717 × 10⁻¹¹ −2.2769 × 10⁻¹⁹   1.7648 × 10⁻²⁴−1.0603 × 10⁻³⁰ M2 1065.0 721.8 Reflection −0.3065   6.6519 × 10⁻¹¹  1.4057 × 10⁻²¹   5.6254 × 10⁻²¹ −1.3690 × 10⁻²⁵ M3 −1186.8 −1583.3Reflection −0.9452 −1.1717 × 10⁻¹¹ −2.2769 × 10⁻¹⁹   1.7648 × 10⁻²⁴−1.0603 × 10⁻³⁰ L3 −687.3 −30.0 Fused 0.0 0.0 0.0 0.0 0.0 Silica LA−460.7 −228.3 Air 0.0   2.1201 × 10⁻⁹    3.8746 × 10⁻¹⁴ −3.5049 × 10⁻¹⁹  1.1333 × 10⁻²⁴ Image

[0090]FIG. 7 shows a ray tracing diagram, and FIG. 8 shows the opticalperformance of the optical system of Example 1 in graphical form.

EXAMPLE 2

[0091] The table below gives exemplary optical prescription for Example2 that uses refractive corrector elements: 1X Optical Design Example #2Optimized For Maximum Slit Width Radius Distance Conic AsphericCoefficients Surface (mm) (mm) Material Constant Y⁴ Y⁶ Y⁸ Y¹⁰ Object ∞420.531 Air 0.0 0.0 0.0 0.0 0.0 L1 −207,468.9 30.0 Fused Silica 0.01.2258 × 10⁻⁹  −1.0071 × 10⁻¹⁵ 5.7191 × 10⁻²²   2.3165 × 10⁻²⁸ L2−161,733.8 2,049.470 Air 0.0 1.1629 × 10⁻⁹  −6.9811 × 10⁻¹⁵ 1.2524 ×10⁻²² 0.0 M1 −2,481.5 −1,171.368 Reflection 0.0 8.4439 × 10⁻¹³ −6.9104 ×10⁻²¹ 3.5714 × 10⁻²⁶ 0.0 M2 1065.0 1,171.368 Reflection 0.0 2.4939 ×10⁻¹² −3.0738 × 10⁻¹⁷ 1.8895 × 10⁻²¹ −3.2975 × 10⁻²⁶ M3 −2,481.5−2,049.470 Reflection 0.0 8.4439 × 10⁻¹³ −6.9104 × 10⁻²¹ 3.5714 × 10⁻²⁶0.0 L3 −161,733.8 −30.0 Fused Silica 0.0 1.1629 × 10⁻⁹  −6.9811 × 10⁻¹⁵1.2524 × 10⁻²² 0.0 L4 −207,468.9 −420.531 Air 0.0 1.2258 × 10⁻⁹  −1.0071× 10⁻¹⁵ 5.7191 × 10⁻²²   2.3165 × 10⁻²⁸ Image

[0092]FIG. 9 shows a ray tracing diagram, and FIG. 10 shows the opticalperformance of the optical system of Example 2 in graphical form.Examples Of Optical Performance Achievable With Proposed 1X OpticalDesign Form Usable Slit Wavefront Numerical Width Quality DistortionTelecentricity Design Aperture (mm) (λ rms) (μm) (mrad) CommentsRequirement >0.10 >9.0 <0.07 <0.6 <0.4 Example 1 0.12 10.0 0.07 0.01 0.2Design optimized for reduced system volume Example 2 0.12 120.0 0.07Design optimized for widest slit width

[0093] The following table shows the improvement in optical performanceachieved by using aspheric surfaces: Optical Performance ImprovementAchieved By Using Aspheric Surfaces In 1X Optical Design Form UsableSlit Wavefront Numerical Width Quality Distortion Telecentricity DesignAperture (mm) (λ rms) (μm) (mrad) Comments Requirement >0.10 >9.0 <0.07<0.6 <0.4 See* below. Example 1 0.12 10.0 0.07 0.01 0.2 (with asphericsurfaces) Example 1 0.12 0.0 0.37 0.01 50.0 This design is (using onlyunable to spherical print the surfaces) required feature sizes for FPDs.

[0094] Note that the exact optical design varies with the FPD size. Inother words, if an FPD twice as large is desired, it is not necessarilythe case that all the original design parameters are simply multipliedby two.

[0095] It will be understood by those skilled in the art that variouschanges in form and details may be made therein without departing fromthe spirit and scope of the invention as defined in the appended claims.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. An exposure system for manufacturing flat paneldisplays (FPDs) comprising: a reticle stage adapted to support areticle; a substrate stage adapted to support a substrate; a reflectiveoptical system adapted to image said reticle onto said substrate, saidreflective optical system comprising a primary mirror including a firstmirror and a second mirror, and a secondary mirror, wherein saidreflective optical system has sufficient degrees of freedom for bothalignment and correction of aberrations when projecting an image of thereticle onto the substrate by reflections off the first mirror, thesecondary mirror, and the second mirror.
 2. The exposure system of claim1, wherein said reflective optical system has 1× magnification.
 3. Theexposure system of claim 1, wherein said reflective optical system has amagnification of between 1× and 10×.
 4. The exposure system of claim 1,wherein said first mirror is aspheric.
 5. The exposure system of claim1, wherein said first and second mirrors are concave, and said secondarymirror is convex.
 6. The exposure system of claim 1, wherein said secondmirror is aspheric.
 7. The exposure system of claim 1, wherein saidfirst, second and secondary mirrors are aspheric.
 8. The exposure systemof claim 1, wherein said secondary mirror is aspheric.
 9. The exposuresystem of claim 1, wherein said reflective optical system has at least14 degrees of freedom.
 10. The exposure system of claim 1, wherein aradius of curvature of said secondary mirror is approximately half ofradii of curvature of said first and second mirrors.
 11. The exposuresystem of claim 1, wherein centers of curvature of said secondary mirrorand said first and second mirrors are nominally coincident.
 12. Theexposure system of claim 1, wherein said exposure system is adapted for42 inch FPD manufacture.
 13. The exposure system of claim 1, whereinsaid exposure system is adapted for 54 inch FPD manufacture.
 14. Theexposure system of claim 1, wherein said exposure system is adapted for60 inch FPD manufacture.
 15. The exposure system of claim 1, whereinsaid exposure system is adapted for manufacture of FPDs between about 40and inches diagonal.
 16. The system of claim 1, wherein said centers ofcurvatures of said first and second mirrors are displaceable to providefine magnification adjustment.
 17. The system of claim 1, wherein saidcenters of curvatures of said first and second mirrors are displaceableto minimize aberrations resulting from magnification adjustment.
 18. Thesystem of claim 1, wherein said centers of curvatures of said first andsecond mirrors are displaceable to correct for any of residual opticalmanufacturing, coating and mount induced errors.
 19. The system of claim1, wherein said first and second mirrors have said substantially thesame curvatures.
 20. The system of claim 1, further comprising aparallel glass plate optically aligned between said reticle and saidfirst mirror.
 21. The system of claim 20, wherein said parallel glassplate includes any of optical glasses, fused silica and optical crystalmaterial.
 22. The system of claim 18, wherein said glass plate has anyof a spherical profile, an aspheric profile, a flat surface and aspherical surface, two spherical surfaces, a spherical surface and anaspherical surface, and two aspherical surfaces.
 23. The system of claim22, wherein any of the spherical and aspherical surfaces have concavecurvatures.
 24. The system of claim 22, wherein any of the spherical andaspherical surfaces have convex curvatures.
 25. The system of claim 1,further including a parallel glass plate optically aligned between saidsubstrate and said second mirror.
 26. The system of claim 25, whereinsaid glass plate has any of a spherical profile, an aspheric profile, aflat surface and a spherical surface, two spherical surfaces, aspherical surface and an aspherical surface, and two asphericalsurfaces.
 27. The system of claim 25, wherein said parallel glass platecompensates for residual aberrations.
 28. The system of claim 1, furtherincluding a first parallel glass plate optically aligned between saidreticle and said first mirror and a second parallel glass plateoptically aligned between said substrate and said second mirror.
 29. Thesystem of claim 28, wherein said first and second glass plates have anyof a spherical profile, an aspheric profile, a flat surface and aspherical surface, two spherical surfaces, a spherical surface and anaspherical surface, and two aspherical surfaces.
 30. The system of claim1, further including a meniscus lens near said glass plate and betweensaid glass plate and said first concave mirror.
 31. The system of claim25, wherein said meniscus lens has an aspheric profile on one or both ofits surfaces.
 32. The system of claim 1, further including a nominallyflat mirror with an aspheric profile positioned at an angle to saidreticle and near said reticle.
 33. The system of claim 1, furtherincluding a nominally flat mirror with an aspheric profile positioned atan angle to said substrate and near said substrate.
 34. The system ofclaim 33, wherein the aspheric profile is different in a vertical andhorizontal axes.
 35. The system of claim 1, further including a firstnominally flat mirror with a first aspheric profile positioned at anangle to said reticle and near said reticle and a second nominally flatmirror with a second aspheric profile positioned at an angle to saidsubstrate and near said substrate.
 36. The system of claim 1, furtherincluding a powered mirror with an aspheric profile positioned at anangle to said reticle and near said reticle.
 37. The system of claim 1,further including a powered mirror with an aspheric profile positionedat an angle to said substrate and near said substrate.
 38. The system ofclaim 1, further including a first powered mirror with a first asphericprofile positioned at an angle to said reticle and near said reticle anda second powered mirror with a second aspheric profile positioned at anangle to said substrate and near said substrate.
 39. The system of claim1, wherein said system is a unit magnification annular optical system.40. The system of claim 1, wherein said first and second mirrors havingcenters of curvature substantially coincident with said secondarymirror.
 41. The system of claim 1, wherein said first and second mirrorsare substantially symmetrically positioned with respect to a center ofsaid secondary mirror.
 42. An exposure system for manufacturing flatpanel displays (FPDS) comprising: a substrate stage adapted to supportan FPD substrate; a reflective optical system adapted to image a reticleonto said FPD substrate, said reflective optical system consisting of afirst mirror, a second mirror, and a third mirror as its poweredreflective elements, wherein said reflective optical system has at least14 degrees of freedom when projecting an image of the reticle onto theFPD substrate by reflections off the first mirror, the second mirror,and the third mirror.
 43. The exposure system of claim 36, wherein saidreflective optical system has 1× magnification.
 44. The exposure systemof claim 36, wherein said first mirror is aspheric.
 45. The exposuresystem of claim 36, wherein said third mirror is aspheric.
 46. Theexposure system of claim 36, wherein said first, second and thirdmirrors are aspheric.
 47. The exposure system of claim 36, wherein saidsecond mirror is aspheric.
 48. The exposure system of claim 36, whereina radius of curvature of said second mirror is approximately half ofradii of curvature of said first and third mirrors.
 49. The exposuresystem of claim 36, wherein said reflective optical system hassufficient degrees of freedom for both alignment and correction of thirdorder aberrations.
 50. The exposure system of claim 36, wherein saidreflective optical system has sufficient degrees of freedom for bothalignment and correction of aberrations.
 51. The system of claim 36,wherein centers of curvatures of said first and third mirrors aredisplaceable to provide fine magnification adjustment and to minimizesaid aberrations resulting from magnification adjustment.
 52. The systemof claim 36, wherein said first and third mirrors have substantially thesame curvature.
 53. The system of claim 36, further including a parallelglass plate near at least one of said reticle and said substrate. 54.The system of claim 46, wherein said glass plate has an asphericprofile.
 55. The system of claim 36, further including a first parallelglass plate optically aligned between said reticle and said first mirrorand a second parallel glass plate optically aligned between saidsubstrate and said third mirror.
 56. The system of claim 36, furtherincluding a meniscus lens near said glass plate and between said glassplate and said first mirror.
 57. The system of claim 49, wherein saidmeniscus lens has an aspheric profile.
 58. The system of claim 36,further including a nominally flat mirror with an aspheric profilepositioned so as to fold an optical axis of the system and positionednear at least one of said reticle and said substrate.
 59. The system ofclaim 36, further including a powered mirror with an aspheric profilepositioned so as to fold an optical axis of the system and near at leastone of said reticle and said substrate.
 60. A unit magnification annularoptical system for flat panel display (FPD) manufacturing comprising: amask having a FPD circuit pattern; a projection optical system forprojecting an image of said mask onto a substrate, said projectionoptical system including a first concave mirror, a second concave mirrorand a convex mirror, said first and second concave mirrors having theircenters of curvature substantially coincident with a center of curvatureof said concave mirror, said first and second concave mirrors beingsubstantially symmetrically positioned with respect to a center of saidconvex mirror, wherein a radius of curvature of said convex mirror isapproximately one-half of radii of curvature of said concave mirror, andwherein an image of the mask is projected onto the substrate byreflections off the first concave mirror, the convex mirror, and thesecond concave mirror.
 61. The exposure system of claim 53, wherein saidfirst concave mirror is aspheric.
 62. The exposure system of claim 53,wherein said second concave mirror is aspheric.
 63. The exposure systemof claim 53, wherein said convex mirror is aspheric.
 64. The exposuresystem of claim 53, wherein said reflective optical system has at least14 degrees of freedom.
 65. The exposure system of claim 53, wherein aradius of curvature of said secondary mirror is approximately half ofradii of curvature of said first and second mirrors.
 66. The system ofclaim 53, further including a parallel glass plate optically alignedbetween said mask and said first mirror.
 67. The system of claim 60,wherein said glass plate has an aspheric profile.
 68. The system ofclaim 53, further including a parallel glass plate optically alignedbetween said substrate and said second mirror.
 69. The system of claim62, wherein said glass plate has an aspheric profile.
 70. The system ofclaim 53, further including a meniscus lens close to said glass plateand optically aligned between said glass plate and said first concavemirror.
 71. The system of claim 65, wherein said meniscus lens has anaspheric profile.
 72. The system of claim 53, further including anominally flat mirror with an aspheric profile positioned so as to foldan optical axis of the system and near said mask.
 73. The system ofclaim 53, further including a nominally flat mirror with an asphericprofile positioned so as to fold an optical axis of the system and nearsaid substrate.
 74. The system of claim 53, further including a poweredmirror with an aspheric profile positioned so as to fold an optical axisof the system and near said mask.
 75. The system of claim 53, furtherincluding a powered mirror with an aspheric profile positioned so as tofold an optical axis of the system and near said substrate.
 76. A unitmagnification annular optical system for flat panel display (FPD)manufacturing comprising: a mask having a circuit pattern for said FPD;a projection optical system for projection of an image of said mask ontoa substrate, said projection optical system including: a first convexmirror, a second convex mirror, and a concave mirror, said first andsecond convex mirrors having their centers of curvature substantiallycoincident with a center of curvature of said concave mirror, said firstand second convex mirrors being substantially symmetrically positionedwith respect to a center axis of said concave mirror, wherein a radiusof curvature of said concave mirror is one-half of radii of curvature ofsaid convex mirrors, and wherein an image of the mask is projected ontothe substrate by reflections off the first concave mirror, the convexmirror, and the second concave mirror.
 77. An exposure system formanufacturing flat panel displays (FPDs) comprising: a source ofelectromagnetic radiation; a reticle stage; a reticle mounted on saidreticle stage; a substrate stage; a reflective optical system adapted toimage said reticle onto a substrate mounted on said substrate stage,said reflective optical system comprising only three powered reflectivesurfaces including a first mirror, a second mirror, and a secondarymirror, wherein said reflective optical system has sufficient degrees offreedom for both alignment and correction of third order aberrationswhen projecting an image of the reticle onto the substrate byreflections off the first mirror, the secondary mirror, and the secondmirror.